The Id2 protein has been implicated in a broad spectrum of cellular processes, including differentiation, cell cycle and tumor progression. Our work discovered that genetic oncogenic changes converge on the activation of Id2 to implement multiple hallmarks of neoplasia, such as uncontrolled cell proliferation, anaplasia and neo-angiogenesis. Although some crucial proteins engaged by Id2 to carry out these functions have been identified (bHLH transcription factors and the pocket proteins Rb, p107 and p130), the mechanisms directing the Id2 activity in normal and tumor cells are still unknown. Important levels of regulation of Id2 are subcellular compartmentalization and the control of Id2 protein stability. We identified the cytoplasmic, actin- associated protein ENH (ENigma Homolog) as a new Id2 protein partner. Accumulation of ENH during neural differentiation and cell cycle arrest is required for cytoplasmic sequestration and functional inactivation of Id2. By combining an array of cell biology and mouse genetics experiments, we will determine the role of the ENH-ld2 pathway in normal development and tumorigenesis in the nervous system. We will also uncover how differentiated tumors restrain their drive towards full-blown anaplasia by retaining active ENH. Recently, we discovered that Id2 is a highly unstable protein that is targeted for ubiquitin-dependent degradation by the Anaphase Promoting Complex/Cyclosome (APC/C) and its activator Cdh1. Degradation of Id2 requires a highly conserved destruction box motif in the Id2 protein. Although several tumors accumulate aberrant amounts of Id2, whether deregulated Id2 operates as an oncogene has never been rigorously demonstrated. We will use mutants of Id2 that are resistant to APC/C-mediated degradation to explore the consequences of instigating deregulated Id2 activity for cell cycle progression, cellular transformation, genomic instability and tumor development in the mouse. By generating mouse models that conditionally express degradation-resistant Id2, we will also unravel the integration of deregulated Id2 with other tumor-inducing mutations for tumorigenesis. The proposed work will define the mechanisms underlying regulation of Id2 and enhance our understanding of the involvement of uncontrolled Id2 activity in human cancer.